Right-click again and select Restore/apstubFreeNote, cdstubFreeNote, or prstubFreeNote based on which stub you are showing and on which you want to add a note to. Initially, the note says "<Your Text Here>".

Right-click on the text and select "Change text" and you can enter whatever text you need.

When you close the window you will be asked if you want to save your changes.